Tuesday, November 13, 2007

The Pale Blue Dot

Image credit: JAXA/NHK

One of the hardest concepts in astronomy for anyone to comprehend (including professional astronomers) is the distance scales we are dealing with in space. Our brains are wired to understand scales of feet, yards, and miles (or centimeters, meters, and kilometers, if you prefer). Most of us can properly imagine relative sizes on these scales. And, if you've done a lot of travelling, even larger scales are imaginable; for example, if you know it takes six hours to drive from San Jose to LA (sans traffic), and an airplane flight of that distance takes an hour, you know roughly how distant a five-hour flight will take you.

But when we go to space, our minds cease to be capable of relating our lives to the cosmic scale. The Moon is 250 thousand miles away; by airplane, that trip would take 18 days. An airplane trip to the sun would take two years (and you thought the 7 hours to Europe was bad -- imagine if you miss your connecting flight!). And the nearest stars are hundreds of thousands of times further away than the sun!

Although it is possible to imagine relative sizes (such as how long an airplane trip might take), our brain just cannot comprehend the true physical distances involved. We've never experienced it.

The above picture was taken by a Japanese moon orbiter called Kaguya. It shows the Earth setting over the south pole of the Moon. That blue globe is our home in high-definition -- yet it is still hard to make out! The land you see in the picture is Australia (upside down) and southeast Asia. Australia is about the size of the United States. Our planet seems so big, and yet, when seen from our nearest celestial neighbor in high-definition, it's hard to make out home at all.

Apollo 8 astronauts took a similar photo in December 1968, a photo which has become one of the iconic views of our planet.

As a side note, the rising and setting of the Earth is due to the orbiting of satellites around the Moon. If you were to stand on the Moon, the Earth would stay in almost exactly the same spot relative to your horizon -- it would never rise or set, though it would go through phases like the Moon. This is because the Moon's rotational speed is identical to its orbital motion (and the same reason why the same side of the Moon is always seen from Earth).

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The Professor Astronomy Blog is based on work supported by the National Science Foundation under grant no. AST-0602288. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation (NSF).